1. Technical Field
The present invention relates to a fixing device used in an office instrument, such as a laser beam printer, a digital printer, a plain paper facsimile, etc., for fixing a toner image onto a sheet by applying heat, and an image forming apparatus including the fixing device.
2. Discussion of the Background Art
In an image forming apparatus using toner for creating a visible image, a fixing device is generally provided to fix a toner image onto a recording medium such as a transfer sheet, etc., as an eternal image. In this fixing device, when passing through a pressure contact section formed between a fixing roller heated and rotated in a prescribed direction and a pressure applying roller rotated by pressure contacting the fixing roller, toner carried on the recording medium is melt and is fixed onto the recording medium. Since toner mainly including plastic has a characteristic of melting at a pressure applying section and sticking to a fixing roller, the toner is generally prevented from sticking to the fixing roller by one of adding wax component to the toner, wrapping the surface of the fixing roller with material having a mold releasing performance, and coating the surfaces of the fixing roller with mold releasing agent, such as silicone oil or the like. Further, a sheet separation mechanism including a separation pick is provided adjacent to the fixing roller to forcibly separate a sheet winding itself around the fixing roller due to the melting toner.
However, since the separation pick sliding contacts the fixing roller, the toner easily accumulates at a contact section, and sometimes contaminates the recording medium. Further, since the separation pick slides over the fixing roller, a sliding mark is put to the surface of the rotation member, thereby decreasing a life, or causing an abnormal image on the recording medium. Since coating the fixing roller with mold releasing agent, such as silicone oil, etc., is refrained while adding wax component to toner recently, the above-mentioned problem becomes significant. In view of the above, even if a separation pick is necessarily used, a non-offensive performance where material of the separation pick does not damage the fixing roller and a non-sticking performance where toner does not firmly stick to the separation pick are demanded.
Fluorocarbon resin is excellent in these performances as described in Japanese Patent Application Laid Open No. 2003-241557. As describe there, a separation pick is integrally formed and made of the fluorocarbon resin entirely. A supporting shaft attached to the separation pick is also molded and made of the resin. However, fluorocarbon resin is inferior at rigidity. Specifically, when an external force is applied such as when sheet jam occurs, the separation pick deforms and damages the fixing roller, or decreases own sheet separation performance. Further, the fluorocarbon resin largely expands and deforms due to heat in comparison with PPS resin or heat resistant resin such as polyimide. Thus, Fluorocarbon resin can't be employed at a leading end of a separation device expected to have a high positional precision. Further, a strike member is sometimes used to create a gap between a separation plate leading end and a rotation member as described in Japanese Patent Application Laid Open No. 2006-11193. When a separation device does not contact a fixing roller and keeps a gap therebetween, the separation device does not damages the fixing roller, and a problem of contaminating an image due to transfer of toner from the separation device to the fixing roller can be avoided even if some of toner firmly sticks to the separation device.
However, the separation plate is made of plate metal and contacts the fixing roller thereby immediately damaging the fixing roller when sheet jam occurs. Further, the separation plate is pressurized and bent by the jammed sheet. To exert a fixing performance within a small space, a sponge member, such as foam silicone material or the like is increasingly used recently as an elastic member of a fixing roller. As a result, a break-into amount of a pressure applying roller increases and a nip width enlarges.
When a fixing roller made of the foam silicon material is utilized, a start up time is decreased, and power consumption is reduced. In such a fixing unit, the above-mentioned problem becomes significant because a strike member breaks into the fixing roller enriched with elasticity due to an external force caused by the sheet jam. As discussed in the Japanese Patent Application Laid Open No. 07-086726, a separation device positioned downstream of a fixing nip includes a separation pick having a peeling off section formed from a substrate made of polyimide coated with fluorine via a primer layer. A supporting section of the separation pick is made of harder resin and is formed by means of insert molding. The peeling off section has a relatively softer than the supporting section. However, since the substrate of the peeling off section is made of polyimide resin, sliding marks are put onto the fixing roller. Especially, in these days, such a problem is significant in a belt type-fixing device using a foam silicon roller enriched with elasticity almost without silicon oil or the like.